Origins of the ASME Fatigue Life Curves Post

How do ASME permissible fatigue curves compare to actual fatigue life results? Can they be used to predict the expected life of equipment?

The graph below is the original data used to create the ASME permissible fatigue life curve for carbon steel alloys[1,2,3]. The black line is the best fit curve though the original data points. I traced the Table 3.F.1 permissible cycle life stress values, the red line, onto the original graph.

Best fit of test results (black line) versus permissible fatigue life. Note that both axis are logarithmic.

The ASME permissible curve is obtained from the original data by adding the following factors of safety: Each point on the ASME line is either 1/2 the value of the original curve above it, or 1/20th the cycle life to the right [Ref 1, page 19]. See the green markups below:

Comparing one data point – the red line should be the most conservative of 20x to the left of, and 2x below the best fit data. It is actually 26x to the left and 4x below.

The 20x safety factor on cycles equals (2x for data scatter) x (2.5x for size effects) x (4x for surface finish and environment) [Ref 1, page 37]. The actual data for the point chosen above – 110 permissible cycles at 200,000 psi has stress divided by 4 (800,000 psi on the graph) and cycles divided by 26 (2860 cycles) – much more conservative than the guideline. Further, when dealing with welds, an additional factor of up to 4x is removed from the permissible cycle life to account for increased stresses inside the weld [4].

These ASME curves are not useful for predicting the cycle life of equipment. They are used to specify a permissible design life which includes a large factor of safety.